Microwave delay apparatus

ABSTRACT

The microwave delay apparatus of this invention consists of an M-type travelling wave tube which employs a photocathode illuminated by a light beam amplitude modulated at the signal frequency as the apparatus for forming and injecting the modulated beam into the drift region. This arrangement permits a laminar beam filling the drift space to be launched into the interaction region where the crossed electric and magnetic fields occur. It eliminates the usual electron gun, its associated beam forming electrodes and the input coupler.

0 United. States Patent [151 3,641,388 Belohoubek Feb. 8, 1972 [54] MICROWAVE DELAY APPARATUS Primary Examiner-Rodney.D. Bennett, .Ir.

Assistant ExaminerRicha.rd E. Berger [72] Inventor. Erwm F. Belohoubek, Kendall Park, NJ. Attorney R L Tompkins and L I. Shrago [73] Assignee: The United States of America as represented by the Secretary of the Navy [22] Filed: Oct. 5, 1965 57 ABSTRACT [21 APPI- N 4 3,275 The microwave delay apparatus of this invention consists of an M-type travelling wave tube which employs a photocathode ilv luminated by a light beam amplitude modulated at the signal {52] US. ?l. ..3 l5/3.5 frequency as the appmms for forming and injecting the [51 1 If. (1...; .1101] 25/34 modulated beam into the drift region. This arrangement WP [58] held of Search ..3lS/39.3, 10, 3.5; 313/65; mits a laminar beam fining the d ift Space to be launched i 330/59 the interaction region where the crossed electric and magnetic fields occur. It eliminates the usual electron gun, its associated I 56] References Cited beam forming electrodes and the input coupler.

UNITED STATES PATENTS 5 Claims, 2 Drawing Figures 3,275,869 9/1966 Feist ..3l3/65 MAGNETIC FIELD B --MODULATED LIGHT SOURCE v PATENTEIIFEB' 8 Ian 3.641.388

ANODE R F. RF. INPUT OUTPUT I I ELECTRON DRIFT I BEAM ANODE l l 5 COLLECTOR DIRECTION OF BEAM FORMING 3 SOLE 7 MAG T HE ELECTRODE 9 NE C PRIOR ART Fig. 1

MAGNETIC /R.F. FIELD B I OUTPUT --MODULA TED LIGHT sOuRCE 2I RGNAL 20 I FIg. 2

In/I/uI-wI /R.

E. F. Belohoubek IJY MICROWAVE DELAY APPARATUS The present invention relates generally to electronic delay devices and, more particularly, to an adjustable microwave delay device of the type employing an electron beam moving in crossed electric and magnetic fields.

Traveling wave tubes have been used in the past as variable delay lines. In one type, the M-type, the electron beam travels through crossed electric and magnetic fields. The microwave signal is transferred to it at an input coupler and then the modulated beam is slowed down and travels at a relatively low velocity over a drift section. It then passes through an output coupler which reestablishes the microwave signal.

The total delay of such an M-type traveling wave tube is the time it takes for the electrons to travel from the beginning of the input coupler to the end of the output coupler. This delay thus depends mainly upon the translational velocity of the electrons in the drift region. Since this velocity is governed by the ratio of the DC electric and magnetic field intensities, the system can perform as a variable microwave delay device by simply changing the magnitude ofthe electric field.

One of the limitation on the maximum delay obtainable with such M-type tubes is due to signal loss brought about by slip between different sections of the electron beam. The major sources of this slip are the potential depression in the beam caused by space charge, mechanical differences in the spacing of the drift plates and variations in the magnetic field within the electron beam. In all of these cases, either the electric or magnetic field in one section of the beam is slightly different from that in another. Consequently, different drift velocities exist in the beam, causing a loss in the microwave signal and deterioration of its wave form.

With these tubes it is also necessary to prevent direct microwave leakage between the input and output couplers. This is done by narrowing the drift region to the point where it is nonpropagating at the operating microwave frequency. I

There are two other problems which influence the performance of these traveling wave delay tubes. One is the adiabatic transition that has to be provided on both sides of the low-velocity drift section. This may involve changes in the beam velocity by a ratio of approximately lOO-tol in order to obtain the required signal delay. The other involves coupling the microwave signal to and from the beam with a minimum of insertion loss.

There are two forms of RF modulation possible with an M- type delay tube. One is space charge modulation, the other, cyclotron wave modulation. Cyclotron modulation has the advantage in that with small signal bandwidth large delays can be achieved in spite of heavy slip conditions. Also, relatively large signal delays can be realized with short drift sections. For small delays or at low frequencies, space charge modulation may be utilized since then the amount of slip is not detrimental and this type of modulation may even lead to a gain condition due to the diocotron effect. Space charge wave modulation with M-type beams, however, is usually more subject to instabilities and noise growth than cyclotron wave modulation.

The present invention provides a new and improved arrangement for forming and injecting modulated electron beams into the drift region of M-type delay types. More specifically, the present invention does away with the usual electron gun and its associated beam forming electrodes. Additionally, it eliminates the input coupler and, in doing so, eliminates one of the adiabatic transitions and the problem of RF coupling between the input and output circuit.

The above components are replaced by a photocathode positioned across the low-velocity drift region of the delay tube and adapted to be illuminated by a light beam that is amplitude modulated at the radiofrequency signal. By focusing on this photocathode such a modulated light beam and by controlling the potential gradient across its surface, a laminar beam filling the drift space can be launched into the interaction region without the excitation of large interfering cycloids. Both space charge wave modulation and cyclotron wave modulation can be achieved by properly choosing the value of the magnetic field and the drift velocity of the beam in the region immediately adjacent to the photocathode.

It is therefore a primary object of the present invention to provide a new and improved variable microwave delay tube of the M type wherein the modulated beam is produced by electrooptical means.

A secondary object of the present invention is to provide a traveling wave tube wherein the conventional electron gun and RF input coupler are .replaced by a photocathode illuminated by a light beam amplitude modulated at the RF frequency.

Another object of the present invention is to provide a microwave delay tube of the M type having effective decoupling between the input and output circuits in the absence of an electron beam.

A still further object of the present invention is to provide an M-type microwave delay device wherein the electron beam is essentially laminar and occupies the entire low-velocity drift section.

A yet still further object of the present invention is to provide a crossed field delay device which can operate with either space charge wave modulation or cyclotron wave modulation.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings wherein:

FIG. 1 shows a conventional M-type microwave tube; and

FIG. 2 illustrates one form of the present invention as applied to a traveling wave tube of the M-type design.

Referring now to FIG. 1 of the drawings which illustrates a prior art arrangement, the M-type traveling delay tube includes an inverted Charles gun for launching the electron beam into the drift region. In this gun the cathode l and its beam forming electrode 2 are arranged slightly above the drift space 3 so that the beam follows a Z-shaped path in entering the RF input coupler 4. This coupler, which has the RF signal applied to one side thereof, may take the form of a capacitively loaded coaxial resonator with the capacitive area acting as the modulation gap for the beam. The modulated beam thus produced is slowed down and moves at a low velocity through the drift section 3 to the output coupler 5 where the RF signal is reestablished and removed. At the far end of the tube the beam strikes the collector 6 which is maintained at a suitable positive potential and connected in an external circuit with the cathode.

In this tube the focusing DC electric field is established between a lower negative plate 7, the sole, and an upper positive plate 8, the drift anode. The DC magnetic field, as shown by arrow 9, is at right angles to this electric field. As mentioned hereinbefore, the signal delay of this apparatus can be adjusted by varying the potential between the sole and the drift anode.

The present invention, as seen in FIG. 2, utilizes an electrooptical system for forming the modulated electron beam. More specifically, the RF signal 20, in one modification, is applied to a solid-state light source, such as the gallium arsenide diode 21, so as to produce a light beam 22 amplitude modulated at the RF signal frequency. This beam is focused by any suitable lens system on a transmission photocathode 23 placed across the low-velocity drift region of the delay tube. The electron stream emitted from the cathode, it will be appreciated, exhibits a density bunching at the signal frequency. By applying a DC potential gradient across cathode 23 equal to that present across sole 24 and drift anode 25, a laminar beam filling the drift tube 26 can be launched into the interaction region without excitation of large interfering cycloids. This gradient may be obtained by utilizing a transparent resistive layer and depositing on this layer the photoemissive material. With this arrangement, the equipotential lines in the drift region end all perpendicularly on the cathode surface and the initial electron velocity is given by V db neglecting the influence of space-charge potential depression.

With the arrangement of FIG. 2, two types of RF modulation are possible. By choosing a magnetic field 27 for which the cycloid wavelength is much smaller than the RF wavelength on the beam, a dominant space-charge modulation may be excited. If, however, the magnetic field has a value close to BF-wh], where w is the signal frequency and 1; is the charge to mass ratio of the electron, and if the cycloid radius R=u,,/-qB is approximately equal to the cycloid wavelength, where u represents the average exit velocity of the electrons from the photocathode, then a dominant cyclotron modulation may be excited. Both of these conditions can be fulfilled without difficulty in the present invention.

The drift region 26 of the tube can be curved so as to eliminated the detrimental effects of slip. Since the emitted beam is essentially laminar and fills the entire drift section, all electrons move with the same angular velocity in this circular drift region. By this means, the system retains the signal coherence.

Adjacent the remote end of the tube, and output coupler 28 is positioned for reestablishing and abstracting the delayed RF signal. Since the beam in this area is generally moving at a high velocity and is consequently relatively thin, this coupler is of reduced height in order to provide good coupling. The tube terminates in a collector 29 which functions in the usual manner.

Since the present invention requires only one coupler, output coupler 28, there is no problem of RF fast wave propagation between the input and output circuits. Consequently, the drift region need not be designed to perfon'n as a nonpropagating RF path. Also, with the present invention, the beam velocity is only changed upward toachieve variable delay. This change is obtained by applying a higher potential to the drift anode V than to the anode V,,. This is in contrast with conventional devices where up and down transitions in beam velocities are necessary. Moreover, in the present invention, the current density and DC current distribution are variable over a wide range by partially making the light impinging on the photocathode, Also, making the light beam on the output side thereof provides a simple means for changing the beam size in the delay tube.

It should be appreciated that the solid-state light source shown in FIG. 2 merely represents one acceptable arrangement for achieving the intensity modulated light beam. Any other equivalent device can be used since the only requirement is that the illumination of the cathode by related to the radio frequency signal which is to be delayed. For example, a regular CW laser may be used with separate light modulator such as KDP or CuCl.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed is: 1. Apparatus for delaying microwave signals comprising, in combination,

a photoconductive cathode; means for illuminating said cathode with a light beam whose intensity is amplitude modulated in accordance with the microwave signals that are to be delayed; means for establishing crossed electric and magnetic fields; means for directing the electron stream emanating from said photoelectrode cathode through said crossed fields in a direction perpendicular thereto; and means for abstracting said microwave signals from said electron stream after it passes through said fields. 2. In a microwave signal delay apparatus, the combination of a pair of spaced, elongated conductors, said conductors serving as opposite boundary surfaces 0 an evacuated drift section of a traveling wave tube; means for establishing crossed electric and magnetic fields between said elongated conductors; electrooptical means for producing at one end of said drift section an electron stream whose density is modulated at a microwave signal frequency; a collector at theother end of said drift section for causing said stream to travel through said crossed fields; and means for abstracting said microwave signal frequency from said electron stream after said stream has passed through said crossed fields.

3. in an arrangement as defined in claim 2 wherein said electrooptical means includes a photocathode which is illuminated with a light beam whose intensity varies in accordance with said microwave frequency.

4. in an arrangement as defined in claim 2 wherein said spaced elongated conductors are curved so that all of the electrons in the stream travel with the same angular velocity through the drift section.

5. In an M-type traveling wave tube delay device for microwave signals, the combination of a pair of spaced elongated conductors,

said conductors serving as a pair of opposite boundary surfaces of an evacuated drift section;

means for producing crossed electric and magnetic fields between said conductors;

a photocathode positioned adjacent one end of said drift section;

a collector at the other end of said drift section;

means for illuminating said photocathode with a light beam whose intensity is modulated in accordance with the microwave signals that are to be delayed whereby an electron beam is launched into the drift region whose density varies in accordance with the microwave signals; and means for reestablishing and abstracting said microwave signals from said electron beam after said electron beam passes through said drift section. 

1. Apparatus for delaying microwave signals comprising, in combination, a photoconductive cathode; means for illuminating said cathode with a light beam whose intensity is amplitude modulated in accordance with the microwave signals that are to be delayed; means for establishing crossed electric and magnetic fields; means for directing the electron stream emanating from said photoelectrode cathode through said crossed fields in a direction perpendicular thereto; and means for abstracting said microwave signals from said electron stream after it passes through said fields.
 2. In a microwave signal delay apparatus, the combination of a pair of spaced, elongated conductors, said conductors serving as opposite boundary surfaces of an evacuated drift section of a traveling wave tube; means for establishing crossed electric and magnetic fields between said elongated conductors; electrooptical means for producing at one end of said drift section an electron stream whose density is modulated at a microwave signal frequency; a collector at the other end of said drift section for causing said stream to travel through said crossed fields; and means for abstracting said microwave signal frequency from said electron stream after said stream has passed through said crossed fields.
 3. In an arrangement as defined in claim 2 wherein said electrooptical means includes a photocathode which is illuminated with a light beam whose intensity varies in accordance with said microwave frequency.
 4. In an arrangement as defined in claim 2 wherein said spaced elongated conductors are curved so that all of the electrons in the stream travel with the same angular velocity through the drift section.
 5. In an M-type traveling wave tube delay device for microwave signals, the combination of a pair of spaced elongated conductors, said conductors serving as a pair of opposite boundary surfaces of an evacuated drift section; means for producing crossed electric and magnetic fields between said conductors; a photocathode positioned adjacent one end of said drift section; a collector at the other end of said drift section; means for illuminating said photocathode with a light beam whose intensity is modulated in accordance with the microwave signals that are to be delayed whereby an electron beam is launched into the drift region whose density varies in accordance with the microwave signals; and means for reestablishing and abstracting said microwave signals from said electron beam after said electron beam passes through said drift section. 